Failure sensing and protection circuit for converter networks

ABSTRACT

This disclosure describes a failure sensing and protection circuit for use with converter networks. The collector-emitter terminals of a power switching transistor are connected in series between a source of power and a converter. A sensing transistor adapted to sense the AC operation of the converter is connected through a current amplifying circuit to the base of the power transistor. In addition, proportional current drive for the power transistor is provided by connecting the current amplifier circuit through a current transformer to sense a signal that is proportional to the current flow in the collector of the power transistor. The drive to the power transistor stops when converter failure occurs because the signal applied to the sensing transistor ends. The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

United States Patent H 1 3,571,662

[72] Inventors T. O. Paine Primary Examiner-Robert K. Schaefer ActingAdministrator of the National Assistant ExaminerWi11iam J. SmithAeronautics and Space Administration with Atl0riteys--R. F. Kempf, E.Levy and G. T. McCoy respect to an invention of; Kenneth J. Jenson,Crystal, Minn. 233 2 ABSTRACT: This disclosure describes a failuresensing and Patented i 1971 protection circuit for use with converternetworks. The collector-emitter terminals of a power switchingtransistor are connected in series between a source of power and aconverter. A

[54] FAILURE SENSING AND PROTECTION CIRCUIT sensing transistor adapted110 561188 the AC operation Of the FOR CONVERTER NETWORKS converter 18connected through a current amplifying circuit to 7 Claims 1 Drawing thebase of the power transistor. In addition, proportional current drivefor the power transistor is provided by connecting [52] US. Cl 317/33,the current amplifier circuit through a current transformer to 321/12sense a signal that is proportional to the current flow in the [51] Int.Cl. H02h 7/10 collector f h power t j The drive to the power [50] FieldofSearch.....

------------------------ 321/11! transistor stops when converter failureoccurs because the 14; 317/33 signal applied to the sensing transistorends.

The invention described herein was made in the per- [56] References cuedformance of work under a NASA contract and is subject to the UNITEDSTATES PATENTS provisions of Section 305 of the National Aeronautics and3,377,540 4/1968 Meyer 321/14X Space Act of 1958, Public Law 85-568 (72Stat. 435; 42 3,439,252 4/1969 Sikes et a1. 321/11 U.S.C. 2457).

POWER m o SOURCE l LOW INPUT lcunnem OUTPUT 3 WW VOLTAGE F CONVERTER----o 02 AC SIGNAL:

PATENTEU HAR23 I97! muzmom W -wzs: w Emma/ INVENTOR KENNETH J. JENSONwomzom 530m on TORNEYS FAILURE SENSING AND PROTECTION CIRCUIT FORCONVERTER NETWORKS BACKGROUND OF THE INVENTION The use of convertercircuits to convert electrical energy from one form to another form orfrom one level to another level are well known. Generally, a DC sourceis connected through transistor power switches to a step-up or stepdowntransformer, as the case may be. The transistor power switches arealternately switched on and off to cause an AC current flow in theprimary winding of the transformer. The secondary winding of thetransformer is connected to a rectifying circuit so that a DC outputsignal of a different level than the DC input signal is provided. Insome environments, if an AC signal is desired, the rectifying circuit iseliminated. Various modifications of this general system are well knownin the art.

One of the problems with prior art converter circuits is that if one ofthe switching transistor fails, undesirable high currents occur whichmay destroy the converter or, in any event, make it unsuitable forfurther use. One prior art system for protecting converter circuits usesa current sensing relay which is connected to sense the output currentof the converter circuit. When a high output current occurs, the relayopens the electrical connection between the power supply and the inputto the converter circuit. The problem with using current sensing relaysystems is that in some environments their mode of operation isundesirable. For example, relays are undesirable for use onboard aspacecraft due to the possibility of arcing across their contacts.Moreover, in the vacuum of space, relay lubricants out gas. In addition,even if the voltage being used is low enough to make arcing anonoccurring phenomenon, relay circuits are bulkier and heavier thandesirable. Hence, it is desirable to provide a solid state switchingnetwork for use in lieu of relay networks to open the primary circuit ofa converter when failure occurs. In addition, because some inverters aredesigned to be utilized with a shorted secondary winding, it isdesirable to sense something other than the current through thesecondary winding to detect transistor failure. Consequently, it isdesirable to provide an apparatus for opening the primary circuit of aconverter when transistor failure occurs that is useful with convertercircuits operating under conditions wherein the secondary winding of theconverter may be short circuited under normal operating conditions.

Therefore, it is the object of this invention to provide a new andimproved failure sensing and protection circuit suitable for use withconverter networks.

It is a further object of this invention to provide a failure sensingcircuit that is adapted to sense the failure of a converter circuitwherein the converter circuit can operate under shorted outputconditions without operating the failure sensing and protection circuitof the invention.

It is a still further object of this invention to provide a failuresensing and protection circuit that is formed of solid state components.

In addition, in some environments, converters operate from low voltagesources, hence, the protection circuit must not have a high voltage dropwhen in operation. Therefore, it is a still further object of thisinvention to provide a failure sensing and protection circuit that issuitable for use with low voltage converters.

SUMMARY OF THE INVENTION In accordance with a principle of thisinvention, a failure sensing and protection circuit suitable for usewith converter networks is provided. The collector-emitter terminals ofa power switching transistor are connected in series between theconverters source of power and the converter. A sensing transistoradapted to sense the AC operation of the converter is connected throughan amplifying circuit to the base of the power transistor. When ACoperation fails, the sensed signal ends and the switching transistor isopened.

In accordance with a further principle of this invention, the amplifyingcircuit is a darlington pair transistor amplifier and is biased by asignal proportionalto the collector current of the power or switchingtransistor.

In accordance with a further principle of this invention, the AC signaldriving the sensing transistor is rectified prior to its application tothe base of the sensing transistor so that unidirectional signals areapplied to the sensing transistor.

It will be appreciatedfrom the foregoing brief summary of the inventionthat a rather uncomplicated apparatus for sensing failure of a converterand preventing the further application of power to the converter from apower source is provided. Due to the fact that AC operation of aconverter is sensed, rather thah just a high current condition, atransistor in the converter must fail before failure of the converter issensed and the power source is disconnected from the converter. Hence,short circuit output currents can occur without interrupting converteroperation. In addition, because the circuit is entirely solid state, itis lighter in weight and less bulky than prior art circuits whereinrelay switching components are utilized. Moreover, because thecollector-emitter terminals of a power transistor ,are connected inseries between the power source and the inverter, only a small voltagedrop occurs across the protection circuit of the invention.

BRIEF DESCRIPTION OF THE DRAWING The foregoing objects and many of theattendant advantages of this invention will become more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawing wherein a preferred em bodiment of the invention isillustrated in partially block and partially schematic form.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION The FIGUREillustrates in partially schematic and partially block form a preferredembodiment of the invention. The block portion of the FIGURE comprises:a DC power source 11; a current transformer T3; and a low input voltageconverter 15. The schematic portion of the FIGURE comprises; three PNPtransistors designated Q1, Q2, and Q3; and NPN transistor designated Q4;four diodes designated D1, D2, D3, and D43; a capacitor designated C;and, five resistors designated R1, R2, R3, R4, and R5.

The positive output terminal of the DC power source 11 is connected tothe emitter of QT, to one end of R1 and to one end of R2. The other endof R1 is connected to the collector of Q1. The collector of Qi is alsoconnected to one input of the low input voltage converter 15. The otherend of R2 is connected to the base of Q1.

The base of O1 is connected to the emitter of Q2 and the base of O2 isconnected to the emitter of Q3. The collector of O3 is connected to thecollector of Q2 and the base of O3 is connected through R3 to thecollector of Q4. The emitter of O4 is connected to the negative side ofthe DC power source 11 which is also ground.

The base of O4 is connected to the cathode of D3. The anode of D3 isconnected through R43 to the collector of Q2. The base of ()4 is alsoconnected to the cathode of D4 and through R5 to ground. The anode of D4is connected to an AC signal output of the low input voltage converter15. The AC signal output could be an AC sense winding on the converterstransformer, for example. Or, the anode of D4 could be connected to thecurrent transformer 13 to sense the AC signal as illustrated by thedashed line. The emitter of O4 is connected to ground or the secondinput of the low voltage input con verter 15.

The capacitor C is connected between the collector of Q1 and thecollectors of O2 and Q3. The collectors of Q2 and 03 are also connectedto the anodes of D1 and D2. The cathode of D1 is connected to one sideof a secondary winding of the current transformer 13 illustrated by thedashed lines. The cathode of D2 is connected to the other side of thesecondary winding of the current transformer 13. The center tap of thesecondary winding of the current transformer 13 is connected to thecollector of Q1. D1 and D2 rectify the AC signal sensed by the currenttransformer and C filters the rectified signal.

The schematically illustrated part of the invention provides theswitching operation that effectively connects the DC power source 11 tothe low input voltage converter 15 with a low voltage drop or isolatesthe DC power source from the low input voltage converter 15. Q1 performsthe power switching function and is driven proportionally to its owncollector current. Proportional drive is effected because the primarywinding of the current transformer 13 is connected in the secondarycircuit of the low input voltage converter 15. The primary winding (andthus the secondary winding) of. the current transformer is directlyproportional to the Q1 collector current because the Q1 collectorcurrent is directly proportional the current flow in the secondarywinding of the low input voltage converter 15. And, because thesecondary winding of the current transformer 13 is connected to thecollectors of O2 and 035, these transistors drive 01 in a manner that isalso proportional. it should be noted that Q2 and Q3 as illustrated inthe FiGURE form the well known darlington pair amplifier circuit. Thedrive path for Q2 and O3 is completed through (Mi. Hence, when 06 isturned on, Q]! is turned on. Q2 and Q3 formed a proportional amplifiercircuit for the 04 signal. 04 (and thus 01) is turned on by current flowthrough either D3 or Dd. Normaily, current flow through D3 initiallyturns 04 on and current flow through D4 sustains Q4 in an on state.

Di provides a path and carries sustaining turn-on power even when theoutput of the low input voltage converter is shorted. It will beappreciated that in some environments the output of a voltage convertermay be shorted during normal operation. For example, the output of a lowinput voltage converter is often shorted when the converter is operatingfrom a thermoelectric source so as to provide cooling for the source. inany event, D 5 provides sustaining turn-on power even when the output ofthe low input voltage converter is shorted. However, when one of theswitching power transistors in the low input voltage converter becomesshorted, no AC signal occurs. Because no AC signal occurs, no power isapplied through Dd to 04. In addition, no power flows through D3 becausea shunt path around D4 through the low input voltage converter wascreated when the transistor became shorted. Hence, neither D3 nor D4apply a signal to the base of Q4 and O4 is biased off. Because O4 isbiased off, O1 is biased off and the DC power source 11 becomes isolatedfrom low input voltage converter. in this manner, the invention sensesinverter faiiure and protects the power source.

it will be appreciated from the foregoing description that the inventionprovides a solid state apparatus suitable for protecting voltageconverter circuits from shorts in their switching components. While theinvention is primarily useful in combination with a low input voltageconverter, because a very small power drop occurs across the protectioncircuit, it is also useful with higher voltage converter circuits.Because the invention senses switch failure as opposed to a shortedoutput, the converter can operate under a shorted output conditionwithout the converter being disconnected from the power source. However,when switch failure does occur, the DC power supply is disconnected fromthe converter.

I claim:

3. A failure sensing and protection circuit for converter networks forconnection between a DC power supply and a voltage converter comprising:

a first transistor having its collector-emitter terminals adapted forconnection between one output terminal of said DC power source and oneinput terminal of said voltage converter;

a second transistor having its base connected so as to sense the ACoperation of said voltage converter; amplifier means connected to thecollector-emitter terminals of said second transistor and to the base ofsaid first transistor to amplify the signal sensed by said secondtransistor prior to its being applied to said first transistor;

current transformer means connected to said voltage converter forsensing the output current of said first transistor; and,

rectifier means connected to the output of said current transformermeans for rectifying the output of said current transformer means andapplying it to said amplifier means.

2. A failure sensing and protection circuit for converter networks asclaimed in claim 1 wherein said amplifier means comprises third andfourth transistors connected in a darling ton pair configuration withthe base of one transistor connected through a resistor to the collectorof said second transistor and the emitter of said fourth transistorbeing connected to the base of said first transistor.

3. A failure sensing and protection circuit for converter networks asclaimed in claim 2 including a first resistor connected in parallel withthe collector-emitter terminals of said first transistor.

4. A failure sensing and protection circuit for converter networks asclaimed in claim 3 including a second resistor and a first diodeconnected in series between the collector of said first transistor andthe base of said second transistor and including a third resistorconnected across the base-emitter terminals of said second transistor.

5. A failure sensing and protection circuit for converter networks asclaimed in claim t including a second diode connected between the baseof said second transistor and said voltage converter to sense the ACoperation of said voltage converter.

6. A failure sensing and protection circuit for converter networks asclaimed in claim 5 wherein said rectifying means comprises:

a third diode connected at one end to one side of the secondary windingof said current transformer means;

a second diode connected at one end to the other side of the secondarywinding of said current transformer means;

the other ends of said diodes connected together and to the collectorsof said third and fourth transistors; and

a capacitor connected between the other ends of said diodes and thecollector of said first transistor.

7. In combination with a DC power supply and a DC to AC power converter,a power transistor having a collector emitter path in series between anoutput terminal of the supply and an input terminal of the converter,means responsive to operation of the converter for deriving a firstsignal having a first value for back biasing the transistor to a cutoffstate in response to a failure of AC power being derived by theconverter, and having a second value for forward biasing the transistorto a conducting state in response to AC power being derived by theconverter, means sensing the level of DC current supplied by thetransistor to the converter for deriving a second signal proportional tothe current level, and means responsive to the first and second signalsfor controlling the magnitude of the current in response to theamplitude of the second signal while the first signal has the secondvalue and for back biasing the transistor while the first signal has thefirst value regardless of the amplitude of the second signal.

1. A failure sensing and protection circuit for converter networks forconnection between a DC power supply and a voltage converter comprising:a first transistor having its collector-emitter terminals adapted forconnection between one output terminal of said DC power source and oneinput terminal of said voltage converter; a second transistor having itsbase connected so as to sense the AC operation of said voltageconverter; amplifier means connected to the collector-emitter terminalsof said second transistor and to the base of said first transistor toamplify the signal sensed by said second transistor prior to its beingapplied to said first transistor; current transformer means connected tosaid voltage converter for sensing the output current of said firsttransistor; and, rectifier means connected to the output of said currenttransformer means for rectifying the output of said current transformermeans and applying it to said amplifier means.
 2. A failure sensing andprotection circuit for converter networks as claimed in claim 1 whereinsaid amplifier means comprises third and fourth transistors connected ina darlington pair configuration with the base of one transistorconnected through a resistor to the collector of said second transistorand the emitter of said fourth transistor being connected to the base ofsaid first transistor.
 3. A failure sensing and protection circuit forconverter networks as claimed in claim 2 including a first resistorconnected in parallel with the collector-emitter terminals of said firsttransistor.
 4. A failure sensing and protection circuit for converternetworks as claimed in claim 3 including a second resistor and a firstdiode connected in series between the collector of said first transistorand the base of said second transistor and including a third resistorconnected across the base-emitter terminals of said second transistor.5. A failure sensing and protection circuit for converter networks asclaimed in claim 4 including a second diode connected between the baseof said second transistor and said voltage converter to sense the ACoperation of said voltage converter.
 6. A failure sensing and protectioncircuit for converter networks as claimed in claim 5 wherein saidrectifying means comprises: a third diode connected at one end to oneside of the secondary winding of said current transformer means; asecond diode connected at one end to the other side of the secondarywinding of said current transformer means; the other ends of said diodesconnected together and to the collectors of said third and fourthtransistors; and a capacitor connected between the other ends of saiddiodes and the collector of said first transistor.
 7. In combinationwith a DC power supply and a DC to AC power converter, a powertransistor having a collector emitter path in series between an outputterminal of the supply and an input terminal of the converter, meansresponsive to operation of the converter for deriving a first signalhaving a first value for back biasing the transistor to a cutoff statein response to a failure of AC power being derived by the converter, andhaving a second value for forward biasing the transistor to a conductingstate in response to AC power being derived by the converter, meanssensing the level of DC current supplied by the transistor to theconverter for deriving a second signal proportional to the currentlevel, and means responsive to the first and second signals forcontrolling the magnitude of the current in response to the amplitude ofthe second signal while the first signal has the second value and forback biasing the transistor while the first signal has the first valueregardless of the amplitude of the second signal.